Measurement Computing PCM-DAS16D/12 - Manual (Page 16)

WARNING
If either the AC or DC voltage is greater than 10 volts, do
not connect the PCM-DAS16x/12 to this signal source. You
are beyond the boards usable common mode range and will
need to either adjust your grounding system or add special
Isolation signal conditioning to take useful measurements. A
ground offset voltage of more than 30 volts will likely dam-
age the PCM-DAS16x/12 board and possibly your computer.
Note that an offset voltage much greater than 30 volts will
not only damage your electronics, but it may also be hazard-
ous to your health.
This is such an important point, that we will state it again. If
the voltage between the ground of your signal source and
your PC is greater than 10 volts, your board will not take
useful measurements. If this voltage is greater than 30 volts,
it will likely cause damage, and may represent a serious
shock hazard! In this case you will need to either reconfigure
your system to reduce the ground differentials, or purchase
and install special electrical isolation signal conditioning.
If you cannot obtain a reasonably stable DC voltage measurement between
the grounds, or the voltage drifts around considerably, the two grounds are
most likely isolated. The easiest way to check for isolation is to change your
voltmeter to it’s ohm scale and measure the resistance between the two
grounds. It is recommended that you turn both systems off prior to taking this
resistance measurement. If the measured resistance is more than 100 Kohm,
it’s a fairly safe bet that your system has electrically isolated grounds.
4.1.5 Systems with Common Grounds
In the simplest (but perhaps least likely) case, your signal source will have
the same ground as the PCM-DAS16x/12. This would typically occur when
providing power or excitation to your signal source directly from the PCM-
DAS16x/12. There may be other common ground configurations, but it is
important to note that any voltage between the PCM-DAS16x/12 ground and
your signal ground is a potential error voltage if you set up your system based
on a common ground assumption.
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